Abstract

The use of opto-electronic devices for ultrafast switching applications is a practical alternative to all electronic devices due to their operating ceiling. The
harnessing of femto-second laser pulses to momentarily switch devices has been the centre of research for over twenty years since the pioneering work of Auston. Though
the design of the electrodes of an Auston switch has not altered to any great effect, the
light absorbing material has been the primary occupation of researchers. The ultrafast
behaviour of these materials is due to the sub picosecond quenching of
photogenerated carriers caused by deep trapping levels pinned between the valence
and conduction bands of the material. The creation of these carrier traps is due to the
presence of non-stoichiometric features in the semiconductor lattice structure. There
have been several attempts to create such material. Originally based on Silicon on
Sapphire materials, the most successful ultrafast photoconductors were found to be
low temperature grown Gallium Arsenide. However this large bandgap material
requires cumbersome gas or titanium sapphire lasers as the pulsed light source. Of
great interest are the Indium Phosphide based materials which can harness the 1550
nm wavelength technology of optical telecommunications where erbium fibre and
solid state mode-locked lasers have been developed which are low cost, compact and
could enable on-chip integration.
One successful approach to achieve 1550 nm absorbing ultrafast
photoconductors has been the use of high energy ion irradiation of Indium Gallium
Arsenide (In0.53Ga0.47As) lattice matched to Indium Phosphide substrates. There has
been research of proton and heavy ion irradiation of 1550 nm wavelength absorbing
materials; but no ultrafast switching devices have been fabricated from lighter ion
irradiation. The advantages of this method are the higher defect concentrations
achievable compared to proton irradiation and the minimising chemical changes to the
material substrates which have been observed with heavy ion irradiation. The
implanted ion chosen in this project was Nitrogen because of its mass and inert
behaviour. In order to demonstrate the ultrafast behaviour of the Nitrogen ion
implanted InGaAs, and to show that it is a practical alternative to LT-GaAs based
devices, a set of ultrafast photoconductive sampling switches were designed,
fabricated and evaluated. This thesis describes the design, fabrication and evaluation of InP based
ultrafast switches capable of sampling waveforms up to 20 GHz. The principle
mechanisms involved in the ultrafast quenching of photocarriers was investigated and
the optimum design for the photoconductive switch determined. An equivalent circuit
of the switch was devised and its expected performance modelled with regard to the
on and off state resistances. Using transform mapping techniques, the switch
capacitance and waveguide dimensions were calculated. The switches were fabricated
using wet etching and metal lift-off techniques prior to evaluation of the pre-irradiated
devices. Once the expected behaviour of the pre-implanted switch had been
characterised, the switches were implanted by high energy nitrogen ions. These
implanted devices were then evaluated and their ultrafast characteristics confirmed.
With a carrier recombination time of 5 picoseconds (FWHM) being measured, this is
the first time that deep nitrogen implantation has been used to create ultrafast InP
based switches.